1. Field of the Invention
The present invention relates to a servo control apparatus for controlling a machine such as a machine tool using a servomotor, and specifically relates to a servo control apparatus that enables a sensorless controlled stop safely in case a sensor malfunction occurs during the operation of the motor.
2. Description of Related Art
During the operation of servomotors (hereinafter also simply referred to as “motors”) used in machines such as machine tools, in general, an alarm condition arises when a malfunction is caused in a sensor, and power to the motor is shut down. When the power is shutdown, the motor is stopped using a dynamic brake (DB) resistor. However, if the motor has a high inertia or is driven at a high speed, its stopping distance is excessive. As a result, the machine may be damaged due to a collision especially in a straight axis direction, thus posing danger.
On the other hand, a controlled stop is effective at shorting the stopping distance. The “controlled stop” refers to stopping the motor under control. However, conventional methods for the controlled stop cannot be used in the event of a sensor malfunction, because sensor information (position, speed, and magnetic pole position) from the motor is required. Therefore, it is desired to develop a sensorless controlled stop method. The “sensorless control” refers to feedback control of the motor based on estimated sensor information.
To detect a sensor malfunction, there are cases where the sensor detects the malfunction or a servo control apparatus that receives the sensor information detects the malfunction. For example, the sensor detects a malfunction due to a pulse miss count or the like detected by an internal circuit of the sensor. On the other hand, the servo control apparatus detects a malfunction due to a pulse dropout or the like occurring after the transmission of the sensor information.
FIG. 1 is a configuration diagram of a general servo control apparatus. The servo control apparatus' 1000 includes a position controller 1004, a speed controller 1005, a current controller 1006, a first coordinate converter 1015, a second coordinate converter 1016, an amplifier 1002, a sensor unit 1001, and a magnetic pole position detector 1003. The position controller 1004 outputs a speed command based on a position command from an upper level control apparatus 1020 and position feedback (FB). To obtain the position FB, an integrator 1014 integrates a motor speed (speed FB) detected by the sensor unit 1001 provided in the vicinity of a motor 1030.
The speed controller 1005 outputs a current command based on the speed command from the position controller 1004 and the speed FB. The current controller 1006 outputs a voltage command based on the current command from the speed controller 1005 and current FB. The current FB is outputted from the second coordinate converter 1016 based on a current value fed back by the amplifier 1002 and a magnetic pole position detected by the magnetic pole position detector 1003. The amplifier 1002 drives the motor 1030 based on the voltage command converted by the first coordinate converter 1015.
Sensor information about the motor 1030 detected by the sensor unit 1001 is fed back to the position controller 1004 as the position feedback (FB), and to the speed controller 1005 as the speed feedback (FB). Moreover, the sensor information is fed back to the current controller 1006 through the second coordinate converter 1016 as the current FB and used therein. In the case of using another scale, the position FB and the speed FB are based on sensor information from the scale.
As a method for sensorless control of permanent magnet synchronous motors, a method using a stator voltage phase is proposed (for example, Japanese Unexamined Patent Publication (Kokai) No. 2011-015601 hereinafter referred to as “patent literature 1”, or “Sensorless Phase-Tracking Control System for Permanent-Magnet Motors” written by Kenji Yamanaka and Tokuo Onishi, published in IEEJ Transactions on Industry Applications Vol. 129, No. 4, pp. 432-437 (2009-4)). Patent literature 1 proposes that by making a stator voltage stably converge to a δ-axis in γδ coordinates (the γ-axial component of the stator voltage Vγ=0), a rotational speed ω is estimated from the δ-axial component of the stator voltage (Vδ), and this allows sensorless control. Patent literature 1 also proposes a correction using a current command to improve a power factor, but its control method in which Vγ=0 is established is the same. Neither method aims at a controlled stop in the event of a sensor malfunction.
As measures for ensuring safety in the event of a sensor malfunction, a method for detecting the sensor malfunction and shifting to sensorless control using a magnetic pole position estimator is proposed (for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-112282, hereinafter referred to as “patent literature 2”). The invention described in patent literature 2 is applicable to machines carrying people such as elevators and cars, and aims at keeping the machines moving to a safe position in a sensorless manner in the event of a failure, to prevent a sudden stop. However, it is preferable to stop machines such as machine tools as quickly as possible to avoid a collision.